Cystic Fibrosis (CF) is a disease characterized by chronic obstruction and infection. In CF, transport of water, sodium and chloride is abnormal. This results in thickened airway secretions that obstruct airways and predispose patients to chronic bacterial infection. Preliminary studies suggest that therapy targeted to this abnormality in ion transport may be beneficial. Recent investigations suggest that the defects in chloride transport can be targeted. INS365 is a compound that may improve transport of chloride and water out of airway cells, therefore hydrating secretions and making them easier to cough out. In addition, secretions are cleared from the airways by hair-like fibers (called cilia) which propel mucus out of the lungs. INS365 makes these hair-like fibers beat more frequently, which may improve the body's ability to clear secretions out of the lung. The primary objectives of this study are to evaluate the safety of escalating single doses of aerosolized INS365 in adult and pediatric CF patients with mild to moderate lung disease and to determine the maximally tolerated dose of aerosolized INS365 in adult and pediatric CF patients with mild to moderate lung disease. The secondary objective of this study is to obtain descriptive information on the effect of INS365 versus placebo on weight of sputum produced following a single dose of aerosolized INS365.